1. Field of the Invention
This invention relates to semiconductor laser driving apparatus to be used for optical information recording and reproducing apparatus and more particularly to a semiconductor laser driving apparatus for superimposing a high frequency.
2. Related Art Statement
Recently, with the development of the information industry, an optical information recording and reproducing apparatus has come to be noted as a large capacity memorizing apparatus.
In an optical information recording and reproducing apparatus using a semiconductor laser (mentioned as an LD hereinafter) as a light source, as a means for reducing the noises generated when the light emitted out of the LD returns to the LD itself and the LD mode competing noises during the operation of the apparatus, there is known an art of superimposing a high frequency on an LD as shown in the respective publications, for example, of Japanese Patent Application Publication No. 9086/1984, Japanese Utility Model Application Laid Open No. 111057/1985 and Japanese Patent Application Laid Open No. 193554/1984.
As described in said publications, in order to reduce noises, it is necessary to make the amplitude of a high frequency current large enough for the laser oscillation to be switched on and off by a high frequency, because, when the amplitude is large enough, the LD will hold a multiplex vertical mode oscillating state.
However, the LD varies greatly in the relation between the operating current and the emitted light amount depending on the fluctuation of the characteristics of the LD itself, the ambient temperature variation and the change with the lapse of time. Therefore, with these condition variations, the LD will be no longer able to keep the on-off state at a high frequency and the noises will not be reduced in some case.
The publication of said Japanese Patent Application Publication No. 9086/1984 discloses no means for determining the high frequency superimposing amount most adapted to the fluctuation of the inherent characteristics of the LD itself, the ambient temperature variation and the change with the lapse of time.
In the publication of said Japanese Utility Model Application Laid Open No. 111057/1985 is disclosed an art of varying the high frequency superimposing amount in response to the size of the driving direct current of the LD. However, at the time of superimposing a high frequency, the LD driving current will vary so greatly that the effect of superimposing a high frequency will depend not only on the size of the driving direct current but also on the characteristic of the variation of the emitted light amount with the LD driving current variation. As this characteristic varies with the fluctuation or the like of the LD characteristic, with the art shown in the publication of said Japanese Utility Model Application Laid Open No. 111057/1985, the high frequency superimposing amount will be in excess or short depending on the fluctuation or the like of the LD characteristic.
Also, in the publication of said Japanese Patent Application Laid Open No. 193554/1984 is disclosed an art wherein the high frequency superimposing effect is measured within a predetermined period during the reproduction of a signal and the high frequency superimposing amount is controlled in response to the measured result. However, the relation between the LD driving current and the emitted light amount is different between the case that there is a returning light to the LD and the case that there is no such light. In said art, as there is a returning light from the recording medium, there is a problem that the high frequency superimposing effect can not be accurately measured under the influence of the difference or the like of the reflection factor of the recording medium.